This invention relates generally to gate-driven semiconductors and more particularly to a gate charge recovery circuit for use therewith. Gate-driven semiconductors widely used in the electronics industry typically include a gate terminal, a reference terminal with respect to which the gate terminal is driven, and a switched terminal. The reference terminal corresponds to the source terminal of a MOSFET or the emitter terminal of an IGBT, for example. Similarly, the switched terminal corresponds to the drain terminal of a MOSFET or the collector terminal of an IGBT. In such semiconductor devices, the driving process involves forcing the gate terminal alternately between a voltage potential to turn the device ON and another voltage potential to turn the device OFF. When the device is ON, the switched terminal exhibits a low impedance characteristic with respect to the reference terminal, whereas when the device is OFF, the switched terminal exhibits a high impedance characteristic with respect to the reference terminal. Since the impedance characteristic exhibited between the gate terminal and the reference terminal is generally capacitive, the process of driving the device from one voltage potential to another is synonymous with forcing a given amount of charge to flow into the gate terminal and out of the reference terminal or vice versa.
In some circuit applications for these devices, it is desirable to detect the current flowing through the switched terminal when the device is ON. This is generally accomplished by inserting a resistor, commonly known as a sense resistor, in series with the reference terminal and detecting the voltage across the sense resistor. This detection method makes use of the approximation that the current flowing out of the reference terminal of the device is representative of the current flowing into the switched terminal of the device. In many applications which use this circuit configuration, the terminal of the sense resistor that is not connected to the reference terminal of the semiconductor device must serve as the ground terminal to provide a common point for one node of a voltage source that is used to drive the device ON.
A general problem with detecting the current flowing through the switched terminal of these gate-driven semiconductor devices is that in the process of turning the device ON, a charge current flows into the gate terminal, out of the reference terminal, and through the sense resistor. As this charge current flows through the sense resistor, a voltage appears across the resistor. Since that voltage is generally present only during a short portion of the full time that the device is ON, it is considered to be a voltage spike. Whereas it is desirable that the voltage waveform across the sense resistor be representative of the current flowing through the switched terminal of the semiconductor device, the voltage spike appearing across the sense resistor represents a deviation from the desired waveform. This voltage spike can present problems in many circuit applications. While attempts have been made to filter the voltage spike, the filtering itself tends to create additional problems.
It is therefore the principal object of the present invention to prevent generation of the voltage spike that would otherwise appear across the sense resistor that is employed to detect the current flowing through the switched terminal of a gate-driven semiconductor device when the device is ON.
This and other objects are accomplished in accordance with the illustrated preferred embodiments of the present invention by providing a capacitor connected between the reference terminal of the semiconductor device and the positive terminal of a driver and a filter resistor connected between the positive terminal of the driver and the positive terminal of a voltage source. As the semiconductor device is turned ON, the capacitor serves to shunt the charge current flowing out of the reference terminal of the semiconductor device back to the positive terminal of the driver, rather than allowing this charge current to flow through the sense resistor. The filter resistor facilitates the shunt function of the capacitor.